1. Field of the Invention
The present invention relates to a hot-dip galvanized steel sheet having a zinc-aluminum alloy galvanized layer (hereinafter referred to as the Zn-Al galvanized steel sheet), and a method for producing the same. More particularly, the present invention relates to a method for preventing an intergranular corrosion of a galvanized layer and propagation of cracks due to intergranular corrosion which occurs when the Zn-Al galvanized steel sheet is stored indoors for a long period of time or in a high temperature- and a high humidity-atmosphere (90.degree. C. or higher and 90% or higher of RH), and for preventing a galvanized layer from peeling from the steel base due to embrittlement of the galvanized layer.
2. Description of the Related Art
Zinc-galvanized steel sheet is the most widely used among surface-treated steel sheets, and demands by users for an enhancement of the qualities of these zinc-galvanized steel sheets grow increasingly stronger. Recently, serious research has been carried out into the development of products, which can meet users demands for enhanced corrosion-resistance, workability, and paintability, produced from sheets galvanized with multi-components, such as Zn-Al.
When a zinc-galvanized steel sheet using an inexpensive base metal of zinc undergoes a secular change when inside a house or is exposed to a high temperature- and high humidity environment, intergranular corrosion occurs. This intergranular corrosion gradually worsens and causes the galvanized layer to become embrittled, and thus to peel away from the steel base. This intergranular corrssion, embrittlement, and peeling of the galvanized layer occurs frequently even in a Zn-Al galvanized layer having an improved corrosion resistance, leading to a serious impairment of the quality of the product.
As an example of an improved resistance against secular peeling, U.S. Pat. No. 4,056,366 discloses a Zn-Al galvanized steel sheet having a galvanized layer characterized by containing from 0.2 to 17 wt % of Al, from 0.02 to 0.15 wt % of Sb, and not more than 0.02 wt % of Pb, the balance being Zn, and a method for producing the same. Further, Japanese Unexamined Patent Publication No. 56-105,447 proposes a zinc alloy for hot dip-galvanizing with an improved intergranular corrosion characterized by a bath composition of from 0.05 to 2.0 wt % of Al, from 0.005 to 1.0 wt % of Mg, and from 0.1 to 1.0 wt % of one or more of Cu and Sb, the balance being Zn and unavoidable impurities. Furthermore, Japanese Unexamined Patent Publication No. 58-177,447 proposes a method for galvanizing a steel sheet by means of a galvanizing bath which consists of from 0.1 to 25 wt % of Al, less than 0.1 wt % of Sb, from 0.05 to 2.0 wt % of Mg, ad not more than 0.01 wt % of Pb, the balance being Zn and unavoidable impurities.
The present inventors proposed, in Japanese Unexamined Patent Publication No. 57-26,155 (Japanese Patent Application No. 55-98,251), a method for producing a zinc galvanized steel sheet with zero spangles, having an improved resistance against secular peeling of a galvanized layer, characterized in that the galvanizing bath consists of from 0.1 to 0.2 wt % of Al, from 0.1 to 0.5 wt % of Sb, and a total of less than 0.02 wt % of the unavoidable impurities, Pb, Cd, Sn, and the like, but excluding Fe, and a balance of Zn.